In an image forming apparatus referred to as a tandem-type, toner images having been fanned in a plurality of tone image fanning sections are transferred onto a transfer body to form a toner image superimposed on the transfer body. The toner image on the transfer body is fixed to form a recorded image or transferred onto a recording medium, followed by being fixed to farm a recorded image.
Toner images having been formed in a plurality of toner image forming sections are transferred onto a moving transfer body in the sequential order from the upstream. Japan Unexamined Patent Publication 2007-241117discloses that a toner to form a toner image having been formed on a transfer body via transfer in the upstream is prevented from being reversely transferred onto the photoreceptor of a toner image forming section of the downstream from the transfer body. Incidentally, in the following description, reverse transfer is referred to as back-transfer.
Initially, back-transfer will be described below.
In FIG. 1a, a toner image T1 on the photoreceptor PC1 of a toner image fanning section G1 on the upstream side is transferred onto a transfer body TM by a transfer member TR1 to form the toner image T1 on the transfer body TM in FIG. 1b. 
In FIG. 1c, in a tone image forming section G2 on the downstream side, a toner image T2 on the photoreceptor PC2 is transferred onto the transfer body TM by a transfer member TR2, but in the transfer member TR2, a part T1b of the toner image T1 which is being formed on the transfer body TM is transferred onto the photoreceptor PC2 of the toner image forming section G2 from the transfer body TM in FIG. 1c and in FIG. 1d, then on the transfer body TM, a toner image T1a and the toner image T2 are fainted. As shown in FIG. 1d, the toner image T1a is formed with a smaller amount of a toner than the toner image T1, resulting in density decrease due to back-transfer in the downstream.
FIG. 2 shows the relationship between transfer current and back-transfer.
The vertical axis represents the toner amount of a toner image on the transfer body TM, and the horizontal axis represents the transfer currents in the transfer members TR1 and TR2. Curve shows the change of the toner amount of a toner image T1 transferred on the intermediate transfer body TM from the photoreceptor PC1 by the transfer member TR1 in the toner image forming section G1 on the upstream side. Curve L2 shows the toner amount of a toner image T1b having been transferred on the photoreceptor PC2 from the transfer body TM by the transfer member TR2 in the toner image forming section G2 on the downstream side.
As shown in the figure, the toner amount of a toner image T1 on the transfer body TM and the toner amount of a toner image T1b on the photoreceptor PC2 change in response to the change of the transfer currents in the transfer devices TR1 and TR2. The toner amount of the toner image T1 increases with the increase of the transfer current and reaches the maximum value at 50 μA, decreasing then with the increase of the transfer current at 50 μA or more. The toner amount of the toner image T1b increases with the increase of the transfer current.
Curve L1 shows changes in response to the change of the transfer current in the transfer member TR1 with respect to a toner image T1 having been formed in the toner image forming section G1, and then changes in response to the change of the transfer current in the transfer member TR2 with respect to a toner image T2 having been formed in the toner image forming section G2 is also shown in the same manse as in curve L1. Therefore, to increase the density of a transferred image of the toner image T2 by increasing the transfer rare of the toner image T2 having been formed in the toner image fanning section G2, the transfer current is preferably allowed to be 50 μA. However, there is produced a problem such that when the transfer current in the toner image forming section G2 increases, the back-transfer amount with respect to the toner image T1 having been formed in the image forming section G1 increases as shown by curve L2.
Therefor, it is necessary to balance ensuring of transfer efficiency and inhibition of back-transfer, For such a balance, appropriate adjustments from the viewpoint of inhibition of color shade change in a color image, ensuring of density, and control of toner consumption are required. Ensuring of density is contrast to control of toner consumption. When an image of high density is obtained at small transfer rate, a toner image with, a large amount of a toner is formed in development and then the amount lost in transfer is replenished. However, when such a method is carried out, the amount of the transfer residual toner is increased and then toner consumption is increased.
In Japan Unexamined Patent Publication 2007-241117, the transfer current in a transfer member on the downstream side is allowed to be smaller than that in a transfer member on the upstream side to prevent back-transfer. Japan Unexamined Patent Publication 2007-241117 does not make clear how the transfer current in the transfer member on the downstream side is set low.
Problems such as inhibition of color shade change, ensuring of density, and control of toner consumption cannot be achieved depending on the method to set transfer current. It is thought that, for example, an appropriate current value in a transfer member on the downstream side is previously determined experimentally, and then the transfer current in the transfer member on the downstream side is uniformly set lower than that in the transfer member on the upstream side regardless of conditions and the image forming mode. However, in the case where the relationship between the transfer current in the transfer member on the upstream side and the transfer current in the transfer member on the downstream side is previously set in this manner, prevention of color shade change of a color image, ensuring of density, and control of toner consumption can be inadequately carried out, and thereby there is noted the problem that when the ambience is changed or the apparatus and material are changed, the color shade and the density of an image are varied. Further, toner consumption is inadequately controlled.
An object of the present invention is to solve the above problems and to realize an image forming apparatus capable of adequately carrying out prevention of color shade change of a color image, ensuring of density, and control of toner consumption.